The anatomy of spice perception involves illusion. We seem to perceive spices both with the senses of taste and smell, but in reality, smell does most of the work. Consider cinnamon . . . . Even with our eyes closed, the smell of freshly baked cinnamon rolls grabs our attention. Sniffing draws the cinnamon volatiles (chemicals that evaporate at low temperatures and make their way into our nostrils as vapors) up into our noses; the volatiles pass through a tiny opening at the top of the nasal cavity called the olfactory cleft. When odorants pass through the cleft, they gain access to the olfactory mucosa, the tissue that contains olfactory receptors. This process is technically called “orthonasal olfaction,” but we commonly call it “smell.”

But there is a second kind of olfaction. When we bite into a cinnamon roll and chew and swallow, the cinnamon volatiles are forced up behind the palate into the nose; because of the backward route by which the volatiles enter the nose, this process is called “retronasal olfaction.” The combination of taste (sweet, salty, sour, bitter) and retronasal olfaction is called “flavor.” Note that we do not use “flavor” as a verb to describe our perceptions of flavor in the same way we use “taste” as a verb to describe our perceptions of taste. To flavor food means to add flavor to food rather than to perceive the flavor of food. But this does not bother us because we use “taste” in everyday conversation to refer to our perceptions of flavor. One of the reasons that we do not notice this linguistic slip is because flavor is perceptually localized to the mouth. This trap caught even Aristotle. He listed olfactory sensations perceived from food in the mouth as tastes.

Why do we experience this illusion of localization? We are not sure, but we know that touch and taste both play roles. The brain knows the route by which an odorant gets to the olfactory receptors. Sniffing may provide the cue that says “orthonasal olfaction.” Oral touch and taste sensations may provide the cues that say “retronasal olfaction.”

In any case, olfactory information goes to different brain areas and is processed in different ways depending on which route was detected. For example, retronasal olfaction can be intensified by taste. Food companies make good use of this intensification. If you market a beverage like grape juice and you would like to intensify the grape flavor of the juice, just add sugar (another reason why we are bombarded with sweetened drinks). Incidentally, supertasters, those individuals with the most taste buds, perceive the most intense tastes, and because of the connection between taste and retronasal olfaction, supertasters also experience the most intense flavors . . . .

Current thinking is that the pleasure we experience from spices is learned. Cinnamon produces pleasure because it was previously paired with experiences our brains are programmed to view favorably (e.g., calories, sweetness of sugar). On the other hand, pair cinnamon with nausea and it will become unpleasant. One of the volatiles in cinnamon, eugenol, is also found in cloves. Cloves and cinnamon do not smell exactly the same, but their odors are similar. Oil of cloves is a natural analgesic and was used by dentists in an earlier era. I associate the odor of cloves with sickness associated with visits to my dentist; I do not share the enthusiasm of those lined up at Cinnabon for the overpowering scents of those calorie-rich rolls. Incidentally, the degree to which learning with one kind of olfaction generalizes to another is not yet clear. Love of cinnamon is learned through retronasal experience but clearly generalizes to cinnamon sniffed. On the other hand, some odors are pleasant with one kind of smell (e.g., cut grass is pleasant when sniffed) but not with the other (I can’t imagine a cut-grass flavor).

The person most responsible for explaining how we learn to love or hate flavors is Paul Rozin . . . .

. . . . Rozin described the “omnivore’s dilemma.” Somehow species like humans (and rats) that consume a large variety of different foods must take in important nutrients and avoid poisons. Rozin and his students have revealed how we do it (Rozin & Hormes, 2009). Our brains note the effects a given food has on us and make us like or dislike the sensory properties of those foods according to its notion of what is good or bad for us. For example, suppose we want to create a food item that will have great appeal. Begin with sources of calories (fat, carbohydrates), add sugar (for its hard-wired effect), and label the mixture with a salient odorant that will endow the item with a retronasal olfactory punch: I give you a brownie. On the other hand, let’s watch an undergraduate on his first alcohol binge get violently ill on screwdrivers. He will likely find screwdrivers distasteful the next day (and possibly for life). Further, the aversion may generalize to orange juice, orange candy, and a lot of other substances flavored with orange. The power of such conditioned aversions has even been used clinically to treat alcoholism . . . .

. . . [Consider one study designed] to explore the affective reactions to odorants in one and two-year olds . . . . The children, seated on their mother’s laps, were allowed to play with toys on a table in front of a picture with holes in it. While the child was engaged with a toy, an odorant was sprayed through one of the holes, and the child’s reaction was rated as pleasant, neutral, or unpleasant by observers in another room viewing the experiment through a one-way mirror. Two odorants were tested that are pleasant to most adults: amyl acetate (pears) and lavender. (To be honest, I don’t know how to describe lavender odor. It’s sold as a spice so there are samples in supermarkets. I suggest you try it.) Two odorants were tested that are unpleasant to most adults: dimethyl disulfide (garlic-like) and butyric acid (vomit-like). There were no significant differences in the reactions of the children to the four odorants. However, by age three, children begin to show preference reactions like those of adults (Engen, 1982; Schmidt & Beauchamp, 1988). The lack of affect at two years along with the appearance of affect over time supports the learning of olfactory affect.

But another issue has yet to be considered: biological benefits of spices. Flavor volatiles in many of the plants we consume are derived from important nutrients; thus, those volatiles could serve as cues to the presence of those nutrients . . . . Further, the subset of plant volatiles that we call spices have been explicitly associated with health benefits. . . .

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The health benefits of spices suggest that we reconsider the possibility of hard-wired liking of at least some spices. For example, is it possible that during evolution some of our ancestors began using turmeric? If turmeric prolonged their lives could this have ultimately contributed to the proliferation of turmeric-likers?

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Harold McGee . . . is most famous for drawing back the curtain and revealing the chemistry behind what we do in our kitchens. . . . McGee [recently] reviewed efforts to find the origin of the burn of chili. He cited work suggesting that higher altitudes seems to produce chilis with greater burn, possibly because the climate at those altitudes may stress the plants, which might make the chilis more vulnerable to attack . . . . Since the burn appears to act as a deterrent to predators, the increase in burn may better repel those predators. This attention to the burn of chilis is a reminder that burn (produced by capsaicin) is a part of what we think of as spices, but it is not a retronasal olfactory sensation; rather, burn is mediated by the trigeminal nerve (which also mediates temperature and touch sensations). Note that the oral burn that probably originated to repel predators can be transformed into a positive sensation in humans. Rozin recently commented that, “many innately negative stimuli . . . become highly desired and emerge as really important foods.”

How does this “hedonic reversal” occur? Some have argued that the biological benefits of chilis (e.g., antimicrobial properties, presence of vitamins A and C) somehow lead to our love of them. Whether or not this is so, children in cultures where chilis are an important part of the diet appear to learn the preference socially; that is, chili initially takes on positive value by association with intake by family and friends. Interestingly, it has proved difficult to induce animals to acquire a preference for chili. Rozin noted that some pets can acquire the preference through the social interaction of pet and owner, but attempts to condition preferences for chili in most animals have met with only modest success. However, one of Rozin’s students, Bennett Galef, was able to condition a mild preference for chili in naïve rats socially by exposing them to rats that had eaten the spice . . . .